A standard developed by the Moving Picture Experts Group (MPEG), known as MPEG, compresses digital video for transmission across a communication medium. Each picture comprises a two dimensional grid of picture elements (pixels), wherein each pixel is associated with a particular spatial location and stores color data associated therewith. Pursuant to the MPEG standard, each picture is subdivided into regions of 16×16 pixels. Each region is represented by a macroblock.
A macroblock stores luminance, and chrominance matrices that are mapped to the 16×16 pixels. The macroblocks are grouped into any number of slice groups or slices. The MPEG standard has been subjected to a number of updates and revisions, resulting in numerous versions. In a version known as MPEG-2, each of the slices contain macroblocks which are all in the same row and contiguous with respect to one another. The foregoing property permits transmission of a bit stream of the macroblocks in raster scan order by transmitting the slices in raster scan order. MPEG-2 also includes syntax indicating where each row in a picture begins. The indication of where each row in a picture begins permits decoding of multiple rows in parallel. Parallel decoding of multiple rows, known as multi-row decoding is especially useful for achieving a decoding rate sufficient for presentation of the video sequence on a video display.
More recent standards, such as the Joint Video Team (JVT) project of ISO-MPEG and ITU-VCEG, known as MPEG-4 Part 10 Advanced Video Coding and as ITU Rec.
H.264, and referred to here as AVC, provide much better compression than MPEG-2, in terms of compressed bit rate for a given level of quality. This makes the more recent standards attractive for commercial deployment. However, such standards are also much more complex to encode and decode than MPEG-2.
The AVC standard has a number of properties that make decoding substantially more complicated. For example, the macroblocks forming a slice are not necessarily spatially contiguous. Slices can include macroblocks that are throughout the entire picture, with macroblocks from other slices groups interspersed therebetween. The foregoing property is referred to here as Flexible Macroblock Ordering and is identified in the AVC standard by having more than one slice group in a picture. Additionally, new rows do not necessarily begin in a new slice and an AVC bitstream does not necessarily indicate where each row starts.
Flexible Macroblock Ordering is problematic because AVC also includes a filtering function known as loop filtering or deblocking. Loop filtering is applied to the entire decoded picture, requiring in general all upper and left neighbors of each macroblock to have been decoded before loop filtering of the macroblock can begin. While it might be possible to decode all the macroblocks of a picture in the order that the macroblocks were received, and then perform loop filtering, doing so prevents concurrent performance of decoding and loop filtering operations, and as a result the performance requirements of decoders of AVC are increased significantly, adding cost. Another problem with sequential performance of decoding and then loop filtering of a picture is that the amount of frame buffer memory required is increased, thereby adding cost to the decoding system.
Macroblocks within a slice include sequences of either variable length codes or alternatively sequences of arithmetic coded syntax elements using the content adaptive binary arithment coding (CABAC) format. In either case, the boundaries between macroblocks can only be determined by parsing substantially all syntax elements from the start of each slice. The boundaries are not assured to fall on convenient boundaries such as byte boundaries. In the case of CABAC, the problem is further aggravated because specific bits may not be assigned to syntax elements, and decoding of each syntax element depends on context established by decoding all previous syntax elements in the same slice.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with embodiments of the present invention as set forth in the remainder of the present application.